The present invention relates to improvements in glass melting furnaces. In particular, the improved glass melting furnace has one or more internal baffles. The baffles confine and direct glass flow within the furnace so that temperature control and fining is improved.
In a glass melting furnace, the time an element of glass remains in the furnace is called the residence time. Residence time has a significant influence on glass quality. As a practical matter, not every element of glass in the furnace has the same residence time. Therefore, the shortest time it takes an element of glass to travel from the inlet of the furnace to the outlet is called the minimum residence time (sometimes hereinafter referred to as the MRT).
In effect, the glass quality is a direct function of the MRT. If the MRT is too short, the glass will not be fully melted or refined. If the MRT is too long, the efficiency of the furnace will be reduced. The factors affecting the MRT must therefore be characterized and quantified before the MRT may be controlled accurately.
One factor of interest is the shortest path from the inlet to the outlet. In a glass melting furnace, the shortest path is not necessarily a straight line from the inlet to the outlet, because molten glass does not usually move uniformly in a straight line.
It is well known that the density of glass is a function of its temperature. The temperature differences in the glass resulting from localized heat input in the furnace cause the hotter, lighter glasses to rise and the heavier, cooler glasses to descend. This movement of glass is commonly called convection.
The shortest flow path from the inlet to the outlet of a furnace is a function of the pattern of rising and descending glass in the furnace. The location and magnitude of heat input, the glass composition and the furnace geometry influence the convection pattern and the MRT. Heat input and location affect the magnitude and direction of glass flow. Glass composition affects convection because infrared transmission is highly dependent thereon, and convection is strongly dependent on the transmission of infrared radiation through the glass. Such transmission is difficult to control because small changes in the glass composition may have a significant affect on infrared transmission. However, the furnace geometry is the most predominant factor affecting convection because it is the most difficult to change once the furnace is built.
If the volume of glass in the furnace is relatively large and unobstructed, the glass is free to move anywhere in the furnace under the influence of the heat input. Unrestricted movement of glass caused by heating is called free convection. If the glass is confined to a relatively small volume, for example a pipe or a duct, the movement of glass is directed or channeled. This is called forced convection.
If the furnace is designed so that the glass is channeled, and thereby undergoes forced convection, the MRT may be more accurately controlled.
If the furnace is designed to melt only one type of glass, the design may be rigidly adapted to produce an optimum MRT for that particular glass. If the furnace is built to melt a variety of glasses, the design must be flexible enough to allow operation of the furnace under various conditions so that each glass composition experiences the correct MRT.
It has been found that certain glasses have improved quality when subjected to relatively high temperatures for a sustained period of time, and also when such glasses are provided with a lengthened flow path, such as by flowing the glasses in paths of limited depths along horizontal surfaces transversely of the shortest path extending axially of the furnace between the inlet and outlet ends thereof. The relatively high temperature helps to put glass batch materials into solution and significantly reduces the problems of unmelted batch stones in the glass. The transverse horizontal paths of limited depth not only allow air bubbles or seeds to agglomerate and move up and out of the glass, but also increase the length of the flow path through the furnace and thereby increase the MRT.
The present invention uses both these techniques, through the use of baffles hereinafter described, to improve glass quality. The baffles, made of a material which is highly resistant to deformation and creep such as molybdenum, also divide the furnace into separate zones, each of which may undergo independent temperature control. This produces a flexible furnace capable of handling a wide variety of glass compositions. Other advantages of the invention will be pointed out in the following description.